The invention relates to a wheel alignment apparatus having measurement heads for determining wheel positions of wheels of a motor vehicle wherein each measurement head comprises at least one camera which is the direct to a measurement target arranged in the field of view of the camera and positioned in a well defined relationship to the wheel of the motor vehicle, and wherein pictures taken by the camera are evaluated by an evaluation unit in order to determine the spatial position of the measurement target and, thereby, of the wheel with respect to the position of the camera and the measurement head respectively.
Such so called contact free wheel alignment apparatus are for example known from EP 0895056, EP 0943890 and DE 19757763 A1. In such wheel alignment apparatus, the measuring heads which serve for determining the wheel positions of the wheels of the motor vehicle on a measurement site, have to be adjusted with respect to each other prior to the actual measurement, and this imposes problems in practice.
In the wheel alignment apparatus known form EP 0895056, the adjustment of the measuring heads with respect to each other is carried out in that the measurement heads are arranged on a common frame on the measuring site such that the relative position of the measurement heads with respect to each other is known. The measurement heads are, however, moveably guided such that accuracy problems arise at the guiding means. Such an arrangement has the deficiency that the distances from the measurement heads to the measurement targets are quite wide such that the resolution of the camera imposes a problem.
In the wheel alignment apparatus known from EP 0943890, the positioning of the measurement heads with respect to each other is carried out in that reference targets are arranged on the measurement site by means of which reference targets the measurement heads are adjusted or positioned with respect to each other. Since the reference targets are arranged on the measurement site in front of the vehicle, the distances from the reference targets to the measurement heads at the real wheels is quite long so that it is difficult to a achieve the required accuracy in calibrating the measurement heads.
In the wheel alignment apparatus known from DE 19757763 A1, the positioning of the measurement heads with respect to each other is carried out in that a separate frame with reference targets is arranged between the measurement heads and the vehicle, where the calibration of the measurement heads is carried out with the aid of the reference targets. In this case, the frame with the reference targets interferes with the installation of the apparatus and during the measurement procedure.
It has already been tried to use the measurement cameras themselves for calibrating the measurement heads, and to arrange reference targets on the measurement heads positioned obliquely opposite to each other with respect to the driving direction of the vehicle. In this case, there is the problem that the camera which serves for carrying out the measurement with respect to the position of the wheel opposite to the camera as well as for calibrating the relative position of the two measurement heads opposite to each other, has to be calibrated twice during each measurement run because of the required changes in the focal length of the camera. If such a camera is also used to provide the relationship to the diagonally opposite measurement head, three calibrations of the camera are necessary, and such calibrations are cumbersome.
In view of there both, it is an object of the invention to provide a wheel alignment apparatus which is designed to operate without a locally fixed reference system and in which the calibration maybe carried out with as few calibration works and hardware components as possible.
For achieving this object, the wheel alignment apparatus of the invention comprises an optical reference system integrated into the measurement heads for calibrating the measurement heads of the wheel alignment apparatus as to the position of the measurement heads with respect to each other. By means of such a references system it is possible in an advantageous way to use mobile measurement heads which are calibrated with respect to the positions to each other prior to the actual measurement.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the references system comprises a reference camera on one of the measurement heads on one side of the motor vehicle and a reference target which is arranged on the opposite measurement head on the same side of the vehicle as well as reference targets on the inside of a forward and a backward measurement head which reference targets are within the fields of view of the measurement cameras of the opposite measurement heads. This arrangement has the advantage that the measurement heads may be freely positioned according to the wheel distance (in driving direction) and the wheel distance transverse to the driving direction, where the measurement targets are positioned in the fields of view of the measurement cameras during the measurement procedure and the measurement and reference targets respectively are positioned in the fields of view of the measurement and reference cameras respectively when no vehicle is present on the measurement site. As compared to a system consisting of the measurement cameras and the measurement targets, only two additional cameras and four reference targets are required. The measurement heads are diagonally exchangeable, and a serf control can be carried out by means of a measurement on the bases of the measurement heads reaching completely around the vehicle. For calibrating purposes, the vehicle has to be removed from the measurement site.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the references system comprises a reference camera on one of the forward measurement heads and a reference target on the opposite measurement head, where the reference camera and the reference target are arranged in front of the vehicle, and wherein the references system further comprises a reference camera on one of the side measurement heads and a reference target on the opposite measurement head on the same side of the vehicle. Hereby, the advantage is achieved that the measurement heads may be position freely according to the wheel distance in the driving direction of the vehicle and to the wheel distance transversely to the driving direction of the vehicle as long as they my detect each other within their respective fields of view. Only three references cameras and targets are required in order to put up the reference system.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein, furthermore, a reference camera is arranged on one of the back measurement heads and a reference target is arranged on the opposite measurement head. Herein, it is advantagoues that the measurement heads may be freely positioned according to the wheel distance in driving direction of the vehicle and the wheel distance transverse to the driving direction of the vehicle as long as they may detect each other within their respective fields of view. The measurement heads are diagonally exchangeable, and self-control of the wheel alignment apparatus is possible by means of the references system which completely surrounding the vehicle as it is the case with the so called eight-sensor wheel alignment apparatus.
According to an advantageous embodiment the invention provides a wheel alignment apparatus comprising an optical beam splitter or mirror system which is arranged between a reference target each and a measurement camera each such alive the reference targets are with in the field of side of the measurement camera. Since, by means of this arrangement, reference cameras are replaced by an optical beam splitter or mirror system, this arrangement has the advantage that a reference camera is saved by means of the beam splitter or mirror system which results in lower costs of the complete apparatus.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the references system comprises one CCD reference camera on each of the side measurement heads which CCD reference camera is directed to the opposite measurement head on the same side of the vehicle, and at least to LED elements each on the opposite measurement head as references targets, and wherein on the forward and/or back reference heads additional CCD reference cameras each and LED elements are arranged on the opposite measurement heads as reference targets. In this arrangement, only ordinary line-cameras and LED elements are required, while the measurement heads are adapted to be freely positioned and diagonally exchanged.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein three LED elements are provided as reference targets. Here, only one CCD-reference camera is required for each measurement head which again serves to reduced costs.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the reference system comprises reference targets on the inside of the measurement heads and an optical reference unit between the reference targets which unit is directed to the reference targets. By means of this arrangement, a central reference unit for calibrating the wheel alignment apparatus is provided which is ready to use after installation and calibrating thereof at any time. The actual measurement heads for measuring the wheel positions may be embodied by relatively simple devices. As to the optical arrangement, the depths of focus of the measurement system and the reference system maybe defined and calibrated separately so that a re-calibration is required only seldom. The measurement heads in this arrangement are again diagonally exchange:
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the optical reference unit comprises four cameras each directed to the reference targets. This arrangement has the advantage that a calibration of the reference system is necessary only once, and that no moving parts are placed in the reference unit.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the optical references unit comprises a reference camera and a mirror or beam splitter system which directs the field of view of the reference camera to the reference targets. By this arrangement, three cameras may be saved as compared to the previous embodiment.
According to an advantageous embodiment the invention provides a wheel alignment apparatus wherein the optical reference unit comprises a ratably mounted reference camera. The actual measurement heads for measuring the wheel positions may be embodied by compatibly simple devices, and the depths of focus of the measurement system and the references system can be defined and calibrated in a simple way. Since only one camera is required for the reference unit, a further reduction of costs as compared to the previous embodiment maybe achieved.
A further understanding of the nature and advantages of the embodiments of the present invention may be realized by reference to the remaining portions of the specification and the drawings.